FIG. 1 shows a block diagram of a conventional battery charging circuit 100. The battery charging circuit 100 includes an alternating current (AC) power source 106, an input rectifier circuitry 110, a flyback converter 112, a pulse width modulation (PWM) controller 114, an isolated converter 116, sensing circuitry 118, a microcontroller (MCU) 120, a control switch 130 and a battery pack 108. The flyback converter 112 includes a main transformer which provides galvanic isolation between input circuitry 102 and output circuitry 104. The flyback converter 112 is operable for receiving a driving signal 140, e.g., a pulse width modulation signal, and for providing a direct current (DC) charging power to the battery pack 108 via the control switch 130. The PWM controller 114 in the input circuitry 102 is operable for generating the driving signal 140 for driving the flyback converter 112. The sensing circuitry 118 can monitor a charging current and a charging voltage, and can feed back information about the charging current and the charging voltage to the PWM controller 114 via the isolated converter 116, e.g., an optocoupler. The microcontroller 120 can monitor the status of the battery pack 108 and can feed back an alert signal to the PWM controller 114 if the battery pack undergoes an undesirable or abnormal condition (e.g., over-current, over-voltage, etc.). The PWM controller 114 can further receive feedback information (e.g., information about charging current and charging voltage, the alert signal, etc.), and can adjust the driving signal 140 to control the charging power according to the feedback information.
Thus, the PWM controller 114 can directly monitor and control an input peak current of the main transformer, such that the main transformer saturation problem can be avoided. However, for purposes of galvanic isolation, the feedback information from the output circuitry 104 may only be transferred by the isolated converter 116, e.g., an optocoupler, which may decrease the accuracy of the feedback information. In addition, the PWM controller 114, the isolated converter 116, the sensing circuitry 118 and the MCU 120 may increase the cost and complexity of the battery charging circuit 100.